In the field of graphic art, an image is printed on a printing plate by using a set of color-separation films prepared from a color original with use of a lithographic film. In general, a color proof is produced from the color-separation films before the main printing (i.e., actual printing operation) so as to check for errors in the color separation process or whether color correction or the like is necessary. The color proof is required to realize high resolution for enabling the formation of a halftone image with high reproducibility and to have capabilities such as high process stability. Furthermore, in order to obtain a color proof approximated to an actual printed matter, the materials used for the actual printed matter are preferably used for the materials of the color proof, for example, the substrate is preferably actual printing paper and the coloring material is preferably a pigment. With respect to the method for producing a color proof, a dry process using no liquid developer is highly demanded.
Accompanying recent widespread use of computerized systems in the pre-printing process (in the pre-press field), a recording system of producing a color proof directly from digital signals has been developed as the dry preparation method for a color proof. These computerized systems are configured particularly for the purpose of producing a color proof having high image quality and by these systems, a halftone image of 150 lines/inch or more is generally reproduced. In order to record a proof having high image quality from digital signals, laser light capable of modulating by the digital signals and sharply focusing the recording light is used as the recording head. Accordingly, the image forming material used with the laser is required to exhibit high recording sensitivity to the laser light and high resolution for enabling the reproduction of high-definition halftone dots,
With respect to the image forming material for use in the transfer image forming method utilizing laser light, a heat-fusion transfer sheet is known, where a light-to-heat conversion layer capable of absorbing laser light and generating heat and an image-forming layer containing a pigment dispersed in a heat-fusible component such as wax or binder are provided in this order on a support (see, JP-A-5-58045 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)) According to the image forming method using such an image forming material, heat is generated in the region irradiated with laser light of the light-to-heat conversion layer and the image-forming layer corresponding to this region is fused by the heat and transferred to an image-receiving sheet stacked and disposed on the transfer sheet, whereby a transfer image is formed on the image-receiving sheet.
Also, JP-A-6-219052 discloses a thermal transfer sheet where a light-to-heat conversion layer containing a light-to-heat converting substance, a very thin (0.03 to 0.3 μm) heat release layer and an image-forming layer containing a coloring material are provided in this order on a support. In this thermal transfer sheet, upon irradiation with laser light, the bonding strength between the image-forming layer and the light-to-heat conversion layer bonded through the heat release layer is weakened and a high-definition image is formed on an image-receiving sheet stacked and disposed on the thermal transfer sheet. The image forming method using this thermal transfer sheet utilizes so-called “ablation”, more specifically, a phenomenon that a part of the heat release layer in the region irradiated with laser light is decomposed and vaporized and the bonding strength between the image-forming layer and the light-to-heat conversion layer is thereby weakened in that region, as a result, the image-forming layer in that region is transferred to an image-receiving sheet stacked on the thermal transfer sheet.
These image forming methods are advantageous in that an actual printing paper having provided thereon an image-receiving layer (adhesive layer) can be used as the image-receiving sheet material and a multicolor image can be easily obtained by sequentially transferring images of different colors on the image-receiving sheet. Particularly, the image forming method utilizing ablation is advantageous in that a high-definition image can be easily obtained. Therefore, these methods are useful for the production of a color proof (DDCP (direct digital color proof)) or a high-definition mask image.
With the progress of DTP environment, at the site using CTP (computer to plate), the intermediate film output process is not performed and instead of proof printing or analogue proof, demands for proof by DDCP system are increasing. In recent years, a large-size DDCP having higher grade and higher stability and having excellent printing consistency is being demanded.
The laser thermal transfer system can provide printing with high resolution and, for example, (1) a laser sublimation system, (2) a laser ablation system and (3) a laser fusion system have been heretofore known, but these systems all have a problem that the shape of recorded dot is not sharp. The laser sublimation system of (1) is disadvantageous in that the approximation to a printed matter is not satisfied due to use of a dye as the coloring material and since this is a system of causing sublimation of the coloring material, the contour of dot becomes indefinite and the resolution is not sufficiently high. On the other hand, in the laser ablation system, a pigment is used as the coloring material and therefore, the approximation to a printed matter is good, but since this is a system of causing scatter of the coloring material, similarly to the sublimation system, the contour of dot becomes indefinite and the resolution is not sufficiently high. Furthermore, the laser fusion system of (3) has a problem that the fused material flows and a clear contour cannot be obtained.
Also, in the printing field, apart from the color printing on white paper, printing on a transparent support is sometimes performed and this is mainly used for package.
At this time, in order to mask the background surface of a transferee material or prevent see-through of the opposite side of a transparent transferee material, it is required to once transfer a white base color on a transferee material and transfer thereon so-called process colors using four colors of yellow, magenta, cyan and black. To cope with such uses, a white thermal transfer sheet is being demanded also in DDCP.
JP-A-2002-86938 is proposing to use a thermal transfer sheet of special color such as white in DDCP, though this publication is silent on uses for package and the like.
However, in the case of white thermal transfer sheet, when the thickness of the image-forming layer is increased to obtain a high masking power, the sensitivity decreases and unless a higher energy is applied, recording cannot be performed, as a result, there arises a problem in productivity or type of recording machine.